1. Field of the Invention
The invention relates to an occupant crash protection device for a vehicle which causes an air bag to instantly inflate in the case of a crash of a vehicle, thereby protecting occupants from being damaged.
2. Description of the Related Art
A conventional occupant crash protection device for a vehicle will be described with reference to Japanese Patent National Publication No. HEI 4-500641 shown in FIG. 7.
Referring to FIG. 7, a plurality of squib firing circuits 1a, 1b, and 1c are connected in parallel between a regulated battery voltage UBatt (not shown) and a ground 2 of a battery mounted on a vehicle. The squib firing circuits 1a, 1b, and 1c include squibs 3a, 3b, and 3c which are connected in series to power transistors T1, T2, and T3 and low resistors R1, R2, and R3, respectively. A known mechanical or electrical acceleration switch 6 is connected between the ground 2 and the common junction of the three resistors R1, R2, and R3 which are connected in parallel.
Since the turn on/off operation of the acceleration switch 6 is conducted in response to deceleration of the vehicle, the switch 6 is closed only when deceleration (or negative acceleration) which is greater than a predetermined value occurs (for example, such as that which may occur in a crash). In the conventional example, therefore, the single acceleration switch 6 is commonly used for the three squib firing circuits 1a, 1b, and 1c. The three squibs 3a, 3b, and 3c are connected to the regulated battery voltage Ubatt (not shown) via a series circuit of a monitor switch 7 and diodes D1 and D2. The diodes D1 and D2 protect the circuits when the battery is connected in an incorrect manner or in the opposite polarity. The circuits are individually tested via the monitor switch 7 so as to check whether the circuits operate normally or not.
The occupant crash protection device is activated when an accident occurs and an acceleration sensor (not shown) operates so that the power transistors T1, T2, and T3 are operated simultaneously or in a synchronized manner via a driver circuit 8 to which a squib firing signal is supplied, and a high voltage output circuit. In this case, when deceleration acting from the outside on the vehicle is sufficiently large and greater than a predetermined level, the acceleration switch 6 is closed so that the current from a single energy storage capacitor E flows through the squibs 3a, 3b, and 3c, thereby causing air bags (not shown) corresponding to the squibs 3a, 3b, and 3c to inflate.
In order to limit the currents flowing through the squibs 3a, 3b, and 3c to a predetermined level, the voltages appearing at the terminals of the resistors R1, R2, and R3 which are connected in series to the squibs 3a, 3b, and 3c are supplied to the inverting (-) input terminals of comparators 9 (in the figure, only one comparator is shown) which function as squib firing current limiting devices, respectively. A constant current is supplied via a resistor R4 from a constant current source 10 to which a stabilized voltage UStab is supplied, whereby a reference voltage is supplied to the noninverting (+) input terminals of the comparators 9. When the voltages of the inverting (-) input terminals of the comparators 9 are higher than the reference voltage supplied to the noninverting (+) input terminals (i.e., the squib firing currents flowing through the resistors R1, R2, and R3 are larger than the predetermined level), the comparators 9 output a control signal to the bases of the respective transistors T1, T2, and T3. This causes the amounts of the collector-emitter currents of the power transistors T1, T2, and T3, i.e., the currents flowing through the squibs 3a, 3b, and 3c are lowered to an acceptable level.
In the above-described occupant crash protection device for a vehicle, however, a resistor of a large allowable power must be connected in series to each of the squibs so that the level of the squib firing current is directly detected. This produces a problem in that the power loss is increased and the capacitance of a backup capacitor must be increased by the value corresponding to the increased power loss.
The currents flowing through the power transistors T1, T2, and T3 and the resistors R1, R2, and R3 are large in amount. For example, a squib firing current of about 2 amperes flows through such an element. When, in order to miniaturize the device, the driver circuit 8, the comparators 9, the power transistors T1, T2, and T3, the resistors R2, R2, and R3, etc. are to be integrated into an IC, therefore, the circuit arrangement must be conducted in view of the amount of generated heat, so that the circuits are not affected by the generated heat, with the result that the IC must have a large-scaled structure for heat radiation. This produces problems in that the production cost is increased, and that the device has large dimensions and hence cannot be practically used.